A light guiding medium, also referred to as fiber optical light guide or optic fiber, can be found in a variety of applications. They include telecommunication optical fiber cable, pyrotechnic optical fiber ignitor, and for modulation of light in order to sense liquid levels. In a typical application, the fiber optical light guide consists of a core material with a refractive index which is slightly higher than that of the so-called cladding material surrounding it. The guided light is confined mainly to the core material, with a small amount of light intensity being present or dissipated in the cladding material. If the cladding material is made sufficiently thin or is completely removed, then the optical wave in the thin cladding or the light near the outer edge of the core interacts with or dissipates into the surrounding medium. In the case of liquid level sensing as exemplified in U.S. Pat. No. 4,287,427, a typical detection system includes an optical fiber, a light source at one end of the fiber, and a detector at the other end. The dissipation into the surrounding medium is utilized to modulate a light signal. When liquid dissipates the guided light, no optical signal is observed by the detector. In the case of telecommunication systems, it is necessary to have specific fiber optic assemblies which are headers, feed-throughs, and connectors. Feed-throughs are devices that could be bolted to the wall of a metal housing. In use, an optical fiber cable would be screwed to the feed-through on the inside wall while another optical fiber cable would be screwed on the outside wall. A connector would be used to put together two links of optical fiber cable. Such fiber optics connectors and feed-throughs are an integral part of the telecommunication industry. In another application pertaining to energetic devices, such devices are used as ignitors, squibs, and detonators. Air bags in automobiles use such energetic components to ignite sodium azide. In the past, energetic components contained a bridgewire. These devices function by passing an electrical signal through lead wires to metal pins which are electrically isolated and sealed within the shell. Welded into the tops of the two metal pins is a thin wire called the bridgewire. The energetic material is placed around and/or next to the bridgewire. During functioning, the electrical signal is passed through the wires via the metal pins to the bridgewire. The bridgewire heats up and ignites the energetic material. However, bridgewire components can be inadvertently set off by lightning, high power radars, radio waves, and the like. Optical fiber components are inherently safer than bridgewire components as they are essentially impervious to lightning, high power radar, radio wave interference, and the like.
Optical fibers may be used in environments which are highly corrosive. Assemblies containing optical fiber are suitable in many applications where electrical assemblies are unsuitable. In view of these hostile environments, the fiber is incorporated into an assembly which effectively isolates or insulates it to minimize or prevent communication of the ambient environment with the environment within which the fiber is disposed. Accordingly, the assembly carrying the fiber must have an essentially hermetic seal which is helium-leak-tight to less than 1.times.10.sup.-8 cubic centimeters per second. U.S. Pat. No. 5,143,531 shows a glass-to-glass hermetic sealing technique to splice links of glass fiber together. A solid glass pre-form is inserted into a cavity of a metal component which is then heated to melt the glass. An end of an optical fiber is then advanced into the molten glass and the entire structure cooled to solidify the glass in sealing engagement with the optical fiber end and the metal cavity. The surface of the resolidified glass may be machined for mating engagement with another component to make, for example, a spliced fiber optic connection. U.S. Pat. Nos. 4,287,427 and 5,143,531 are incorporated herein by reference in their entirety.
Although the technique described in U.S. Pat. No. 5,143,531 has certain advantages in that it does not require use of any foreign sealing material such as solder, the components of the assembly are subject to being highly stressed or exhibit cracks, either as a result of fabrication or during use.